In the recent years, there has been an increased awareness on the environmental impact of large amounts of lubricants and industrial fluids entering the environment due to leakage or spillage. If the lubricant has little biodegradability, accumulation of these lubricants will result in environmental pollutions.
The industrial lubricating oil currently in daily use largely based on mineral oils that are highly toxic to the environment. Mineral oils based lubricating oils are not readily biodegradable by microorganisms. Recent studies also indicate that these oils are carcinogenic. Consequently, there is an obvious need for lubricating fluids which are based on renewable natural resources, and at the same time environmentally friendly.
Vegetable oils are the potential candidates to replace conventional mineral oil based lubricating oils. Natural vegetable oils are non-toxic, exhibit a ready biodegradability, good lubricity and cause fewer health problems such as allergies. Rapeseed oils and sunflower oils are the two most common vegetable oils currently used in the formulation of biodegradable lubricants in Europe.
Meanwhile, the use of natural or modified vegetable oil as hydraulic fluid has been disclosed in several patents. Konishi et al. (U.S. Pat. No. 6,300,292) describes the use of rapeseed oil as a suitable base for hydraulic fluid. Honary (U.S. Pat. No. 5,972,855) used slightly modified soybean oil as a base oil to produce hydraulic fluid. Before this, the soybean oil is partially hydrogenated followed by a winterisation process. Lawate et al. (U.S. Pat. No. 5,538,645) used genetically modified high oleic vegetable oils as base fluid.
Transesterified oil is another group of base oil used by researchers in the formulation of environmentally acceptable hydraulic fluid. Kodali (U.S. Pat. No. 6,278,006) transesterified vegetable oil with a short chain fatty acid ester to produce a base suitable for industrial lubricant. Lamsa (U.S. Pat. No. 5,885,946) described a process of transesterifying vegetable oil with a lower alkanol to produce an alkyl ester and then the alkyl ester is further transesterified with a polyol. Lamsa (U.S. Pat. No. 5,885,946) provided a method for preparing a base for synthetic lubricant similar to U.S. Pat. No. 5,885,946 but using the enzymatic route.
Synthetic ester is yet another group of base fluid used. Hartley et al. (U.S. Pat. No. 6,054,420) described the preparation of a biodegradable lubricant or functional fluid using synthetic ester of polyhydric alcohol. Hartley et al. (U.S. Pat. No. 5,880,075) also described the use of the ester of oxoisodecyl alcohol with iso-stearic acid in combination with polyol ester in the preparation of synthetic biodegradable lubricant. Watanabe et al. (U.S. Pat. No. 5,607,907) disclosed the use of TMP (trimethylolpropane) esters of caprylic or capric acids adjusted with stearic acid as hydraulic fluid.
However, synthetic esters have the problem of higher prices and synthetic esters excellent in biodegradability have the disadvantage of being inferior oxidative stability. On the other hand, the vegetable oil which is excellent in biodegradability and superior in respect of lower prices, but they have poor thermal, oxidative and hydrolytic stability. In general, poor cold temperature properties of natural oil can be improved by increasing the degree of unsaturation in the natural oil. However high degree of unsaturation contributes to poor oxidative stability. For these reasons, the natural oils may only be used in the less severe applications. Rapeseed oil and castor oil, for instance, have been used in lubricants in limited specific applications.
Palm oil, even though possesses good biodegradability and lubricity and better oxidative stability compared to a highly polyunsaturated oil, is not chosen because of its poor cold temperature fluidity.
In order to improve on the properties of vegetable oils, the glycerine molecule of the vegetable oil can be substituted with a hindered alcohol. Usually alcohols without β-hydrogen, such as neopentyl glycol, trimethylol propane and pentaerythritol, are used. The vegetable oil is first hydrolysed to its fatty acids and glycerol. The hydrolysed fatty acids are then re-esterified with a hindered alcohol. This improves the thermal, oxidative and hydrolytic stability of the oil significantly without affecting much on the biodegradability. This new range of product is generally known as synthetic ester.